Trump’s early approach holds promise if backed with a sustained, and sustainable, commitment.

Can the U.S. succeed in Afghanistan? Not without a sustained, and sustainable, commitment. President Trump’s decision to give Defense Secretary Jim Mattis the authority to add several thousand more U.S. troops to the 8,400 currently deployed is encouraging—but only if it is a first step in a comprehensive approach.

Army Gen. John Nicholson, commander of U.S. and allied forces in Afghanistan, should also receive greater leeway in the use of U.S. and NATO air power. And officials should remain open to the possibility of...